For recognizing a handwritten character such as "R" in FIG. 1, concavities 1 and 2 and a hole are important characteristics. Therefore, convexity and concavity are important characteristics of a configuration. The smallest convex configuration including a configuration to be processed (original configuration hereinafter) is called convex hull. By subtracting the original configuration from the convex hull, concavities and holes are extracted, for characterizing the original configuration. However, the convex hull cannot easily generated in the strict meaning, in the conventional method. In the conventional method, top points of convex hull are successively connected by straight lines. In a digital image, a straight line is different from a line in the mathematical meaning, because a digital image consists of dispersed pixels and mathematical continuous line cannot always be drawn. The digital line may be an approximation and pixels outside of the convex hull may be took as pixels on the line. When a pixels outside of the convex hull is took, the configurations generated by the above subtraction represent other characteristics than the characteristics to be extracted. Usually a graphic processor is used for drawing lines, so lines are drawn according to the algorithm of the processor. It is not guaranteed that a suitable convex hull for recognition is always generated.
For evaluating a concavity, several characteristic parameters are proposed; chord length of a concavity, inner peripheral length of a concavity, concavity ratio etc. Unless concavities are exactly extracted, calculation of these parameters become in vain.